/usr/share/doc/libobjc-6-dev-mips64-cross is in libobjc-6-dev-mips64-cross 6.3.0-18cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-6-cross-base-ports
/usr/share/doc/gcc-6-cross-base-ports